The present invention relates to a process for hot working a duplex ferrous alloy, e.g. duplex stainless steel. In particular, the present invention relates to a process for carrying out superplastic deformation of duplex stainless steel.
A duplex alloy which comprises a ferritic (alpha) phase and an austenitic (gamma) phase exhibits markedly improved properties such as strength, toughness, and weldability so that the industrial demand therefor has recently been increasing. However, a duplex alloy has also been known as a hard-deformable material, since it contains a duplex phase, i.e., a ferritic phase+austenitic phase.
In the prior art, in order to improve the hot workability of such a material, much effort has been made towards decreasing impurities such as sulfur, oxygen, etc. which are harmful to hot workability. Therefore, it has been possible to form products of a simple shape, such as plates, pipes, and simple forging products.
However, it is quite difficult to produce articles having a complicated shape, such as pipe joints, valves, etc. These products, therefore, have been produced by means of machining or casting, and such articles are accordingly expensive.
Superplastic deformation has recently been used to produce non-ferrous material products of a complicated shape. Regarding ferrous materials, especially duplex ferrous alloys, it has been reported that superplastic behavior can be observed in duplex ferrous alloys, such as duplex stainless steel containing large amounts of Cr, Mo, and Ni, which is well known as a hard-deformable material. For example, "Metal Science" May 1976, pp. 182-188, discloses a strain rate at which superplastic behavior is observed, although it is extremely low, e.g. for usual material 10.sup.-4 -10.sup.-5 S.sup.-1 1, and thus too low for economical industrial use. In addition, the presence of Ti(C,N) is essential to the superplastic deformation. Furthermore, this reference does not suggest anything about the presence of a finely dispersed ferrite+austenite duplex microstructure, which is effective to achieve superplastic deformation according to the present invention.